Assessment of the toxicity of methyl parathion and its photodegradation products in water samples using conductometric enzyme biosensors

An assessment of the toxicity of methyl parathion and products of its photodegradation in water samples was performed using conductometric biosensors based on thin films planar electrodes and immobilized cholinesterases. All photodegradation experiments were carried out using a quartz reaction cell equipped with a 125 W high pressure mercury arc lamp as source of radiation. The photodegradation kinetics can be described as a first-order degradation curve C = C-0 e(-kt), where C-0 = (9.1 +/- 0.1) X 10(-5) M, k = 0.026 +/- 0.001 min(-1) and t(1/2) = 26 min. The high performance liquid chromatography (HPLC) was used to identify and quantify methyl paraoxon and 4-nitrophenol as main methyl parathion photoproducts. It was shown for the first time by using conductometric enzyme biosensor that the inhibition effect on the acetyl cholinesterase (AcChE) activity increases dramatically as soon as the photodegradation of methyl parathion begins. Methyl paraoxon is about 10 times more toxic than methyl parathion. The toxicity curve does not exactly follow the appearance of methyl paraoxon. The results obtained could be explained by a strong synergistic effect of methyl parathion and methyl paraoxon on the enzyme activity. (C) 2002 Elsevier Science B.V. All rights reserved.